There are many applications for light modulator devices that have high spatial and time resolution and high brightness, including applications in displays of information for education, business, science, technology, health, sports, and entertainment. Some light modulator devices, such as digital light-mirror arrays and deformographic displays, have been applied for large-screen projection. For white light, light modulators such as the reflective digital mirror arrays have been developed with high optical efficiency, high fill-factors with resultant low pixelation, convenient electronic driving requirements, and thermal robustness.
Macroscopic scanners have employed mirrors moved by electromagnetic actuators such as “voice-coils” and associated drivers. Micro-mirror devices have used micro-actuators based on micro-electro-mechanical-system (MEMS) techniques. MEMS actuators have also been employed in other applications such as micro-motors, micro-switches, and valves for control of fluid flow. Micro-actuators have been formed on insulators or other substrates using micro-electronic techniques such as photolithography, vapor deposition, and etching.
A micro-mirror device can be operated as a light modulator for amplitude and/or phase modulation of incident light. One application of a micro-mirror device is in a display system. In such a system, multiple micro-mirror devices are arranged in an array such that each micro-mirror device provides one cell or pixel of the display. A conventional micro-mirror device includes an electrostatically actuated mirror supported for rotation about an axis of the mirror into either one of two stable positions. Thus, such a construction serves to provide both light and dark pixel elements corresponding to the two stable positions. For gray scale variation, binary pulse-width modulation has been applied to the tilt of each micro-mirror. Thus, conventional micro-mirror devices have frequently required a high frequency oscillation of the mirror and frequent switching of the mirror position and thus had need for high frequency circuits to drive the mirror. Binary pulse-width modulation has been accomplished by off-chip electronics, controlling on- or off-chip drivers.
Conventional micro-mirror devices must be sufficiently sized to permit rotation of the mirror relative to a supporting structure. Increasing the size of the micro-mirror device, however, reduces resolution of the display since fewer micro-mirror devices can occupy a given area. In addition, applied energies must be sufficient to generate a desired force needed to change the mirror position. Also, there are applications of micro-mirror devices that require positioning of the mirror in a continuous manner by application of an analog signal rather than requiring binary digital positioning controlled by a digital signal. Accordingly, it is desirable to minimize a size of a micro-mirror device so as to maximize the density of an array of such devices, and it is desirable as well to provide means for positioning the micro-mirror device in an analog fashion.
Micro-electromechanical systems (MEMS) are systems which are typically developed using thin film technology and include both electrical and micro-mechanical components. MEMS devices are used in a variety of applications such as optical display systems, pressure sensors, flow sensors, and charge-control actuators. MEMS devices of some types use electrostatic force or energy to move or monitor the movement of micro-mechanical electrodes, which can store charge. In one type of MEMS device, to achieve a desired result, a gap distance between electrodes is controlled by balancing an electrostatic force and a mechanical restoring force.
MEMS devices designed to perform optical functions have been developed using a variety of approaches. According to one approach, a deformable deflective membrane is positioned over an electrode and is electrostatically attracted to the electrode. Other approaches use flaps or beams of silicon or aluminum, which form a top conducting layer. For such optical applications, the conducting layer is reflective while the deflective membrane is deformed using electrostatic force to direct light which is incident upon the conducting layer.
More specifically, MEMS of a type called optical interference devices produce colors based on the precise spacing of a pixel plate relative to a lower plate (and possibly an upper plate). This spacing may be the result of a balance of two forces: electrostatic attraction based on voltage and charge on the plate(s), and a spring constant of one or more “support structures” maintaining the position of the pixel plate away from the electrostatically charged plate. One known approach for controlling the gap distance is to apply a continuous control voltage to the electrodes, where the control voltage is increased to decrease the gap distance, and vice-versa. However, precise gap distance control and maintenance of reflector parallelism may be affected by several factors, including material variations between support structures, misalignment in lithographic methods used in device fabrication, undesired tilt of reflecting surfaces, and other variations. While various light modulator devices have found widespread success in their applications, there are still unmet needs in the field of micro-optical light modulator devices.